1. Field of the Invention
The present invention relates to a superconducting fault current limiter, more particularly to a fault current limiter utilizing the superconductivity of a ceramic high-temperature superconductor the superconducting state of which is switched to a resistive state when applied with a fault current beyond a critical value.
2. Discussion of the Prior Art
In recent years, there has been proposed an inductive current limiter for protection of a circuit breaker, and a transformer in power networks from fault currents caused by a short-circuit or thunderbolt. When applied with fault currents, the inductive current limiter provides a high impedance which limits the fault currents to restrain an electric load acting on the circuit breaker and transformer below a threshold level. In U.S. Pat. No. 5,140,290 granted to Dersch, there is disclosed a high power inductive current limiter of this kind which is composed of an induction coil with at least one winding through which current flows, a cylindrical body made of a ceramic high-temperature superconducting material concentrically arranged within the induction coil, and a core made of a soft magnetic material of high permeability and concentrically disposed within the cylindrical body. In normal operation, the superconductivity of the cylindrical body shields the magnetic field of the induction coil completely from the core (Meissner effect), and impedance of the induction coil is maintained at a very low level to minimize loss of the electric power. When a fault current flows through the induction coil due to a short-circuit or thunderbolt, the superconductivity of the cylindrical body disappears and the impedance of the induction coil reaches its maximum, current-limiting value.
To maintain the superconductivity of the cylindrical body in normal operation, the inductive current limiter is immersed in cooling liquid such as liquid nitrogen. In the conventional inductive current limiter, however, the cylindrical superconductive body is surrounded by the induction coil and is in contact with the cooling liquid only at its inner peripheral surface. For this reason, the cooling efficiency of the superconductive body becomes insufficient, particularly when the superconductive body has a large thickness. Furthermore, the superconductive body is greatly affected by eddy current losses in the iron core and joule losses caused by the resistance of the induction coil since it is disposed between the iron core and the inductive coil, resulting in a decrease of the critical value of the fault current limiter.
Since the conventional fault current limiter is entirely immersed in the cooling liquid to cool the superconductive body, the cooling device becomes large in size. In actual use of the fault current limiter, the induction coil is heated by its self-resistance when applied with transport current, and the iron core is also heated by the flow of eddy current caused by a magnetic field acting thereon after transition to the resistive state. As a result, the cooling liquid is heated by heat generation of the induction coil and iron core. In case the cooling liquid fails due to the heat generation, there will occur an excessive increase in volume of the cooling liquid, resulting in rapid increase of the internal pressure of the cooling device. To avoid such a problem, it is required to provide a pressure release mechanism on the cooling device. This results in a complicated construction of the cooling device and an increase of the manufacturing cost.
Additionally, the self-resistance of the iron core becomes small when the iron core is cooled. This causes an increase of the eddy current in the iron core after transition to the resistive state. Thus, the magnetic field in the iron core is reduced by the eddy current, and the relative permeability of the iron core becomes small. Accordingly, the iron core does not serve to increase the self-conductance of the induction coil after transition to the resistive state.
It is therefore, required in commercial applications of the fault current limiter to enhance the cooling performance of the cooling device in a reliable manner and to provide the cooling device in a small size at a low cost.